Touch screens are visual displays with areas that may be configured to detect both the presence and location of a touch by, for example, a finger, a hand or a stylus. Touch screens may be found in televisions, computers, computer peripherals, mobile computing devices, automobiles, appliances and game consoles, as well as in other industrial, commercial and household applications. A capacitive touch screen includes a substantially transparent substrate which is provided with electrically conductive patterns that do not excessively impair the transparency—either because the conductors are made of a material, such as indium tin oxide, that is substantially transparent, or because the conductors are sufficiently narrow that the transparency is provided by the comparatively large open areas not containing conductors. For capacitive touch screens having metallic conductors, it is advantageous for the features to be highly conductive but also very narrow. Capacitive touch screen sensor films are an example of an article having very fine features with improved electrical conductivity resulting from an electroless plated metal layer.
Projected capacitive touch technology is a variant of capacitive touch technology. Projected capacitive touch screens are made up of a matrix of rows and columns of conductive material that form a grid. Voltage applied to this grid creates a uniform electrostatic field, which can be measured. When a conductive object, such as a finger, comes into contact, it distorts the local electrostatic field at that point. This is measurable as a change in capacitance. The capacitance can be measured at every intersection point on the grid. In this way, the system is able to accurately track touches. Projected capacitive touch screens can use either mutual capacitive sensors or self capacitive sensors. In mutual capacitive sensors, there is a capacitor at every intersection of each row and each column. A 16×14 array, for example, would have 224 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.
WO 2013/063188 by Petcavich et al. discloses a method of manufacturing a capacitive touch sensor using a roll-to-roll process to print a conductor pattern on a flexible transparent dielectric substrate. A first conductor pattern is printed on a first side of the dielectric substrate using a first flexographic printing plate and is then cured. A second conductor pattern is printed on a second side of the dielectric substrate using a second flexographic printing plate and is then cured. The ink used to print the patterns includes a catalyst that acts as seed layer during subsequent electroless plating. The electrolessly plated material (e.g., copper) provides the low resistivity in the narrow lines of the grid needed for excellent performance of the capacitive touch sensor. Petcavich et al. indicate that the line width of the flexographically printed material can be 1 to 50 microns.
Flexography is a method of printing or pattern formation that is commonly used for high-volume printing runs. It is typically employed in a roll-to-roll format for printing on a variety of soft or easily deformed materials including, but not limited to, paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, glass, glass-coated materials, flexible glass materials and laminates of multiple materials. Coarse surfaces and stretchable polymeric films are also economically printed using flexography.
Flexographic printing members are sometimes known as relief printing members, relief-containing printing plates, printing sleeves, or printing cylinders, and are provided with raised relief images onto which ink is applied for application to a printable material. While the raised relief images are inked, the recessed relief “floor” should remain free of ink.
Although flexographic printing has conventionally been used in the past for printing of images, more recent uses of flexographic printing have included functional printing of devices, such as touch screen sensor films, antennas, and other devices to be used in electronics or other industries. Such devices typically include electrically conductive patterns.
To improve the optical quality and reliability of the touch screen sensor film, it has been found to be preferable that the width of the grid lines be approximately 2 to 10 microns, and even more preferably to be 4 to 8 microns. In addition, in order to be compatible with the high-volume roll-to-roll manufacturing process, it is preferable for the roll of flexographically printed material to be electroless plated in a roll-to-roll electroless plating system.
After the touch screen sensor film has been printed and plated in roll-to-roll format, electrical testing is typically performed to eliminate defective touch screen sensor devices. For compatibility with high-volume roll-to-roll manufacturing processes, it is preferable to electrically test the touch screen sensor devices while they are still in roll format and prior to separation from the roll. This is particularly true if there are subsequent roll-to-roll processes such as application of protective liners or films.
The use of machine vision systems for aligning test probes with test pads of a device to be electrically tested is known. For example, U.S. Pat. No. 5,442,299 to Caggiano, entitled “Printed circuit board test fixture and method,” and U.S. Pat. No. 5,321,351 to Swart et al., entitled “Test fixture alignment system,” disclose test systems for printed circuit boards where alignment of the test probes is performed with reference to the sensed positions of fiducial marks formed on the printed circuit board.
With increased emphasis on device miniaturization it is desired to reduce the size and spacing of test pads, thereby requiring tighter alignment tolerances for the test probes. In addition, for roll-to-roll fabrication of devices on a flexible web of substrate, there can be distortion due to web tension or slippage during printing for example. The distortion can cause errors in the placement of the fiducial marks relative to the test pads. For a large area device or a large area set of devices being tested simultaneously, such placement errors of the fiducial marks can decrease the reliability of test probe alignment with the test pads, especially if the test pads are small and closely spaced. A test probe that misses its pad can result in the erroneous detection of an open circuit such that a false failure is recorded.
What is needed is a machine vision system that is capable of reliable alignment of test probes of an electrical test fixture to test pads of a device, even if there is distortion in the placement of fiducial marks relative to test pads.
Alignment of test probes for roll-to-roll electrical testing is an example of a more general problem for systems configured to perform an operation on an article, where the article includes a plurality of fiducial marks and a set of features of interest. The problem is that if fiducial marks are provided for determining the positions of the set of features, and if there is a variable placement error between the fiducial marks and the set of features, then the determination of the location of the fiducial marks alone can provide unreliable information regarding the positions of the set of features. The subsequently performed operation can sometimes miss its intended position when the variable placement error is sufficiently large. What is needed is a system that is capable of accurate determination of the positions of features of interest, even if there is uncertainty in the placement accuracy of fiducial marks associated with the features of interest.